This application is based upon and claims the benefit of Japanese Patent Application No. 11-235693 filed on Aug. 23, 1999, the contents of which are incorporated herein by reference.
1. Field of the Invention
This invention relates to a refrigerating cycle using carbon dioxide as refrigerant.
2. Description of the Related Art
Lubricating oil (refrigerating machine oil) for a compressor in a refrigerating cycle is generally mixed with refrigerant to be supplied to a sliding part in the compressor. Polyalkylene glycol (PAG) and polyol ester (POE) are widely known as lubricating oil for the refrigerating cycle using, as refrigerant, flon such as 134a. 
In a hermetic electric compressor driven by an electric motor, refrigerant must be introduced into a housing of the electric motor for cooling the electric motor. This compressor is difficult to use PAG as lubricating oil because PAG mixed with 134a (flon) has extremely small electric insulation resistance. Therefore, when the refrigerating cycle using 134a (flon) as refrigerant includes such a hermetic electric compressor, generally, POE having electric insulation resistance larger than that of PAG is used as lubricating oil. Incidentally, PAG is generally used in a refrigerating cycle equipped with an open type compressor which is provided separately from a driving device such as an electric motor.
On the other hand, a kind of refrigerating cycle uses carbon dioxide as refrigerant and includes an accumulator for separating lubricating oil from refrigerant discharged from an evaporator and for supplying the separated lubricating oil to a suction side of a compressor together with gaseous phase refrigerant. This kind of refrigerating cycle is referred to as an accumulator cycle below. When POE having high compatibility relative to carbon dioxide is used as lubricating oil in the accumulator cycle, a large amount of liquid phase refrigerant that is dissolved in lubricating oil can be sucked into the hermetic electric compressor together with lubricating oil. Accordingly, the hermetic electric compressor is brought into a liquid compressed state, and the efficiency of the compressor is lessened.
The present invention has been made in view of the above problems. An object of the present invention is to provide an accumulator cycle including a hermetic electric compressor, which can prevent efficiency of the compressor from being lessened.
According to the present invention, in a refrigerating cycle using carbon dioxide as refrigerant, lubricating oil for a hermetic electric compressor contains as a main component one of polyalkylene glycol (PAG) and poly (vinyl ether) (PVE).
Accordingly, in state where lubricating oil is mixed with refrigerant (carbon dioxide), the lubricating oil can exhibit high electric insulation resistance that causes no problems on a practical use. Further, since lubricating oil can have lower compatibility with respect to carbon dioxide than POE, a large amount of liquid phase refrigerant is very difficult to be sucked into the compressor while being dissolved in lubricating oil. Therefore, the efficiency of the compressor is not lessened.